The most economical method of drilling a well bore is to use conventional open hole drilling techniques to drill an appropriate diameter hole with standard drilling fluid compositions. However such conventional methods are frequently not suitable where "abnormally" pressured formations may be encountered by the well bore during drilling. Such conditions frequently lead to sticking the drill string or risking the blowout of the well by high pressure gas or oil in the over-pressured zone. Such sticking may occur by inadvertent fracture of a lower pressured zone above or below the high pressure zone. Accurate prediction of pore pressures that may be expected along the length of a drilling well, especially exploration wells, has traditionally been a difficult industry problem. Where abnormal pressures are known to exist, or may be found unexpectedly along the length of such well bores, accurate prediction of the depth at which such pressures will be encountered may be critical to the economic success of the drilling operation. The particular problem presented in these situations is that it is generally necessary to run several different diameters of concentric well casings from the surface of the earth to points above and below such high pressure zones. This permits control of the well bore pressure by drilling fluid alone through potentially oil productive zones. The cost of individual well casings from the surface to the intended well depth represents high economic risk to drill the well. If the well bore and casing are too large, the drilling cost per foot of depth is substantially higher. If the bore hole is too small, the result may be that the diameter of the well casing and the bore hole is too small to accommodate the necessary number of drill casings to control the well. This may lead to drilling a well so that it fails to reach the intended depth objective, due to inadequate casing diameter to accommodate enough concentric casing strings. On the other hand, unnecessary well casing programs may interfere with use of conventional well testing or well logging methods to evaluate potentially productive formations along the well bore.
It is a particular object of the present invention to predict more accurately pore pressures that may be encountered in drilling the proposed well, and particularly in undrilled areas, including wildcat or stepout wells, where subsurface data is incomplete or not readily available. An estimate of the pore pressures in a well bore to be drilled at a selected well location is obtained by utilizing seismic data collected and recorded down to depths extending through both an area below a proposed well site and below or adjacent to a drilled well near the proposed well. In both areas, time-amplitude seismic "traces" having a common mid-point (CMP) between a plurality of pairs of sources and detectors are recorded for a multiplicity of different source-detector distances or offsets, each pair having the same common mid-point. Such separate traces are selectively corrected and stacked or combined to correct for "move-out" (to compensate for different distances between each pair of sources and detectors after reflection from various depths along the mid-point). Such traces are also corrected for dip of the reflecting horizons and other errors due to difference in geometry and geology between the sources and detectors. Such combined traces yield so-called "stacking velocities" over the various subterranean geological intervals underlying both the adjacently drilled and proposed well location. The drilled well is selected as close as possible to the given or proposed drill site. However, the only information contained in the combined seismic traces are time and amplitude; that is, the time required for a seismic wave to travel from a source to a seismic discontinuity, which then reflects the wave back to a detector, and the amplitude of the reflected wave. The actual depth of each reflector in such traces must be converted to depth-amplitude by relying entirely upon a knowledge of the velocity of each interval of the rock sequences through which the seismic wave has traveled before it is imaged by the seismic trace.
Such seismic time and amplitude records or "traces", indicate only the average of all strata through which the seismic energy has passed, from the source to a reflector and back to a detector. Where the depth of the strata acting as reflectors are shallow, say a few thousand feet, and the bedding planes are not complex, few of the amplitudes represent multiple reflections, and such velocities correlate well with the true depths of such strata. However, at greater depths of, say 5,000 to 14,000 feet, such recorded traces velocities may change with depth, and geometry of the reflecting beds, as well as many factors depending upon physical or chemical structure, or both. Thus, based on the summed interval velocities of all strata, the depth of such seismic reflectors, as evidenced by the amplitude spikes, may vary in actual depth over several tens or even hundreds of feet. Accordingly, a particular difficulty in previously known methods of predicting pore pressures at given depths under an exploratory well from such seismic data alone lies in the lack of detailed information as to the actual velocities of portions, or intervals, of the geological column, through which the seismic waves travel.
It has been proposed, (and commonly used in field operations) to calibrate pore pressures derived from measurements in an adjacent drilled well to pore pressures based on an interval transit time log derived from measurements based on of seismic traces recorded at the earth's surface from common mid-points adjacent the same well. The well pore pressures are generally based on empirically developed mathematical relations between interval transit times recorded as sonic logs, either directly generated, or synthesized, from electrical logs, conductivity logs, or density logs recorded in the well, and pore pressures measured on cores or chips recovered from drilling fluid from known depths in the drilled well. These calculated values are then directly related to true depth of strata to determine the interval velocity of strata penetrated by a well bore. Thus, actual pore pressures at selected depths are convertible (from interval transit times for some waves) to pore pressures extending over the same depth as the adjacent seismic data trace.
Based on such measured pore pressures in the adjacent drilled well, it has been further proposed to use a similar stacked seismic trace at a proposed well site to calculate similar pore pressures from interval transit times measured on a synthetic sonic log constructed from such seismic trace. However, calculations of pore pressures from selected interval transit times also require a knowledge of the "normal" trend of pore pressures over the same depth intervals. Because such detailed knowledge, required to draw a normal trend line, is frequently not available from the adjacent well, a normal trend line is generally drawn arbitrarily to match the general appearance of the seismic trace. Furthermore, because such a normal trend line in fact depends on age and lithology of the geological column, as well as pore pressures, I have found that the true values of pore pressures derived from a seismic trace and an assumed "normal" trend line may be vertically displaced over significant depths of from several feet up to several hundred feet. This is due to the need at each depth to use both the "measured" values established by the seismic curve and "normal" values of the overburden, or geostatic pressures established by the trend line at the same depth to calculate a pore pressure at that depth. Accordingly, the pore pressures assigned to the seismic curve, to a large extent, depend upon the accuracy of a normal trend line selected arbitrarily. Accordingly, such derived pore pressures at a given depth are not sufficiently accurate to develop a reliable drilling program of casing diameters and drilling fluid weight, or density, to adequately control well pressures at critical points during drilling.